1. Field of the Invention
This invention relates broadly to devices and methods for combining bulk optical devices with optical fiber transmission, and more specifically relates to an optical fiber device having an attachment to an optical device package with improved modes of adjustment, and relates to methods for its assembly.
2. Discussion of the Related Art
The use of bulk optical devices in the midst of optical fiber transmission requires the interconversion of approximately collimated optical beams with fiber-optic transport. This interconversion employs a connecting assembly usually involving the use of collimators that have to be accurately aligned to the collimated beam path and then fixed into permanent position as part of the device packaging process. Typically, the connecting assembly is a pre-assembled assembly including a fiber termination, with one or more fiber ends, and a lens that is positioned and fixed relative to the fiber termination prior to attachment to the device package, so that an optical beam of approximately constant cross-section, a collimated beam, can be optically coupled to or from the fiber.
The tutorial FIG. 1 shows variations in how a collimated beam can emerge from an optical device package through a hole 12 in the package wall 11. The beam may pass through at different places and at different angles. Specifically, beam #3 passes through the hole highest, and beam #2 lowest. Beam #1 is nearly orthogonal to the wall 11, but beam #3 is tilted downward the most. Such variations can easily occur due to variation in optical parts and variation in placements of the internal optics and/or the other pre-assembled collimator assemblies, located at other openings (not shown) of the optical device package.
In placing a pre-assembled collimator assembly (hereinafter, PCA) near to the wall to couple well to each of these three illustrated beams, we need to place the PCA at the right position (height, and depth into the paper) and with a suitable tilt to match that of the actual beam. For a coordinate system, let the z-axis be orthogonal to the package wall, approximately parallel to the beam direction. Let the x-axis be perpendicular to the plane of the paper, and the y-axis in the plane of the paper, pointing up. Small angular tilts can be described as small rotations about the x-, y-, and z-axes respectively.
Although the complete specification of the orientation of a solid in space takes six parameters (for six degrees of freedom), the placement of a collimator to couple well to the beam is not very sensitive to displacement along the z-axis, nor to rotation about the z-axis in the absence of polarization sensitivity. Thus, the main concern is for the other four degrees of freedom and the corresponding four parameters to control.
Tutorial FIG. 2 shows the relative insensitivity of loss to axial displacement, that is, translational displacement along the z-axis. Moving up to three centimeters in z results in a loss of 0.06 dB, less than two percent loss. The specific curve 21 in FIG. 2 is for a beam at 980 nanometers (nm), which has been expanded conventionally in cross-section to an approximate diameter of 0.4 mm to reduce the effects of diffraction within the interior optics of an optical device package, of which the package wall is wall 11 in FIG. 1.
Tutorial FIG. 3 shows in curve 31 that for a translation of the collimator in x or y, lateral motion, movement of 0.1 mm, 100 micrometers, from optimum causes a loss of about one dB, or about 20 percent loss. The specific curve 31 is also for a conventionally expanded beam of approximately 0.4 mm diameter at 980 nm.
In curve 41 tutorial FIG. 4 shows for lateral beam tilts a sensitivity that is considered relatively great. Lateral beam tilts are angular tilts of the beam about the x-direction or the y-direction. A tilt of less than three minutes of arc from optimum will result in a loss of about one dB. The specific curve 41 is also for a conventionally expanded beam of approximately 0.4 mm diameter at 980 nm.
In summary, axial displacement, displacement in z, needs control only by accurate manufacture of the package to its optical design, but for the four sensitive parameters, greater precision is needed.
According to the invention, a connecting assembly has a contact feature that contacts a first surface of a washer-like structure, and a second surface of the washer-like structure contacts the wall around the opening in the wall of the optical device package. The first and second surfaces are shaped differently and, preferably, each provides at least two directions of adjustment by relative motion of the contacting surfaces, prior to attachment.
According to the method of the invention, the washer-like structure is moved on its area of contact with the wall, and the connecting assembly contact feature and the washer-like structure are moved relatively on their area of contact until both the washer-like structure and the connecting assembly are centered on the beam path.
According to a feature of the invention, attachment is then made at both surfaces of contact.